1. Technical Field
The present invention is directed generally to electromechanical machines, and, in particular, to a system and a method for current regulation in the field-weakening operation of permanent magnetic (PM) machines.
2. Description of the Related Art
In the control of inverter-driven permanent magnet (PM) machines, field-weakening is often used to lower the inverter voltage rating for a given application. That is, without field-weakening the inverter manufacturer would have to use components rated to handle higher levels of voltage for that given application. This would undesirably add incremental costs to the drive system. Field-weakening may be accomplished by configuring the machine windings to provide a greater torque per amp ratio, and thus achieve a lower base speed for a given torque load. During high speed operation, the phase current may be applied to the machine windings in advance of the phase electromotive force. To control the electromotive force, the d-axis current is decreased inversely with speed.
Above the base speed of the machine, i.e., where the line-to-line electromotive force voltage due to the magnets has become greater or equal to the source voltage, a field-weakening current is applied to the machine in order to maintain torque. The flux created by this current is in opposition to the rotor flux, and thus reduces the effective electromotive force seen by the inverter. It should be stressed that this current should be carefully regulated to a target value set by the commanded torque and rotor speed. Failure to control the current to an appropriate value will likely result in the application of excessive voltage to the inverter, which is undesirable.
It is known to provide a flux weakening algorithm by the use of a number of look-up tables to produce the reference Q-axis and D-axis currents. However, the use of look-up tables requires the creation of numerous and cumbersome data structures within the look-up tables themselves to handle all possible situations in the system and its environment.
As further background, as alluded to, operation in the field-weakening region of PM machines may involve commanding the required D-axis current (Ids) as a function of speed. This can be implemented as a look-up (noted above) or as a mathematical function. Another disadvantage, however, of this approach is that if the magnetic characteristics of the PM machine change due to rotor temperature variations and/or the DC-link voltage change, the look-up table or function may no longer be valid.
This may cause the current regulator to exceed the voltage limit, which results in the loss of machine control, which is undesirable.
Accordingly, there is a need for a control system that minimizes or eliminates one or more of the above-mentioned shortcomings.